Absence of Intracellular Potential Gradients in Amphibian Embryos

Abstract

THE developmental biology literature contains many references to the existence of bioelectric potential gradients in embryonic and regenerating systems^1–5 and, because the polarity of regenerating hydroids and planarians can be altered by application of electric fields^6–8, a role in the control of spatial patterns of cellular organization has been assigned to potential differences which have been measured in vivo. With few exceptions, the experimental evidence in support of the existence of potential gradients is based on measurements made with extracellular electrodes applied to the surface of the particular organism or tissue under consideration. The reliability of some of these measurements is difficult to assess because the potential values quoted tend to be rather small, in the range 10⁻⁶ to 10⁻² V, while information concerning, the magnitude of errors arising from electrode junction potential asymmetries is often not available. If extracellular measurements reflect a true intracellular potential gradient in the organism, electrodes introduced directly into the interior of those cells situated at opposite ends of its major axis should record a larger gradient provided that the internal resistance is greater than that of the external fluid. This is because potential values recorded extracellularly are shunted by the conducting medium in which the tissue is usually bathed.